The present invention generally relates to a method of operating a production or assembly line and, more particularly, to a method of operating a production line including self-propelled carriages or modules for transferring workpieces along machines in work zones for assembling and machining the workpieces.
The automobile industry has some of the most advanced automated production lines in the world. The production lines include automatic workpiece feed means or conveyors which carry the workpieces through a series of work stations or zones having industrial robots to effect various operations such as assembling, coating, welding, and other processes. The conveyor typically includes self-propelled carriages or cars which individually transfer the workpieces along a guide rail or track along the production line. The self-propelled carriages are desirable because they can start-up and stop automatically and can easily increase speed into or out of a work zone having automated equipment.
Flow of the carriages along the production line requires each of the assembly operations to have a fixed processing time. The carriages, therefore, move at a fixed speed and are spaced by a fixed pitch to provide the required processing time for each of the operations along the assembly line.
FIGS. 1A to 1E illustrate a work zone 10 of a production line 12 wherein a plurality of carriages 14 are entering from a preceding work zone 16 and are exiting to a subsequent work zone 18. In FIG. 1A, the carriages 14 are continuously moving at a fixed operational speed and a fixed operational pitch in a forward direction as indicated (left to right as shown in FIG. 1A).
FIG. 1B illustrates a first trouble spot or stoppage 20 which has occurred along the production line 12 wherein one of the carriages 14 has been stopped for some reason such as a production or equipment problem. In the illustrated example, the last carriage 14 (carriage No. 0) in the subsequent work zone 18 has been stopped. When the carriage 14 (carriage No. 0) is stopped, each carriage 14 upstream of the stopped carriage 14 (carriage No. 0) must also stop so that the fixed operational pitch is maintained. The stoppage of one carriage 14, therefore, stops the entire portion of the production line 12 which is upstream of the stopped carriage 14 (carriage No. 0). It can be appreciated that stopping the entire upstream portion of the production line 12 results in lost production.
When the stopped carriage 14 (carriage No. 0) is restarted, each of the upstream carriages 14 are also immediately restarted so that the fixed operational pitch is maintained as illustrated in FIG. 1C. As the carriages 14 forwardly move along the production line 12 after restarting, they continue to move at the fixed operational pitch and the fixed operational speed. As shown in FIG. 1D, when an upstream carriage 14 (carriage No. 12) is stopped due to a second trouble spot or stoppage 22, each carriage 14 upstream of the stopped carriage 14 (carriage No. 12) must stop so that the fixed operational pitch is maintained. Each carriage 14 downstream of the stopped carriage 14 (carriage No. 12), however, continues to forwardly move along the production line 12. Therefore, a space or gap 24 forms between the stopped carriage 14 (carriage No. 12) and the forwardly adjacent carriage 14 (carriage No. 11) which continues to proceed along the production line 12.
When the stopped carriage 14 (carriage No. 12) is restarted, each of the upstream carriages 14 are also immediately restarted as shown in FIG. 1E. After restarting, the carriages 14 all forwardly move along the production line at the fixed operational pitch and the fixed operational speed. It is noted however, that the gap 24 which was developed by the second stoppage 22 remains and progresses along the production line with the carriages 14 at the fixed operational speed. It can be appreciated that the gap 24 results in lost production by the production line 12. In the illustrated example, the second stoppage 22 resulted in a gap 24 equivalent to one missing carriage 14. It should be noted, however, that stoppages of longer duration result in gaps equal to additional missing carriages 14.
FIGS. 2A to 2D show the effect of continued operations upstream of the stoppage when there is already a gap 24 located upstream of the stoppage. In FIG. 2A, the carriages 14 are continuously moving at the fixed operational speed and the fixed operational pitch in a forward direction as indicated (left to right as shown in FIG. 2A). A gap 24a is present between two of the carriages 14 which was formed downstream of a previous stoppage. In the illustrated example, the gap 24a is located in the preceding work zone 16 between the second and third carriages 14 (carriage Nos. 12 and 13). The illustrated gap 24a is equivalent to two missing carriages 14. It should be noted, however, that the gap 24a could be equal to a fewer or greater number of missing carriages 14.
FIG. 2B illustrates the first trouble spot or stoppage 20 which occurred along the production line 12. When the carriage 14 (carriage No. 0) is stopped, each carriage 14 upstream of the stopped carriage 14 (carriage No. 0) must also stop so that the fixed operational pitch is maintained. The stoppage of one carriage 14, therefore, stops the entire portion of the production line 12 which is upstream of the stopped carriage 14 (carriage N. 0). Therefore, the size of the gap 24a does not change.
Each carriage 14 downstream of the stopped carriage 14 (carriage No. 0) continues forward along the production line 12. FIG. 2B illustrates the production line 12 at the instant when the carriages 14 preceding the stopped carriage have progressed a distance equal to twice the fixed pitch. Therefore, another gap 24b has formed downstream of the stoppage 20.
FIG. 2C illustrates the production line 12 at the instant when the carriages 14 preceding the stopped carriage have progressed a distance equal to four times the fixed pitch. The size of the upstream gap 24a again has not changed because all of the carriages 14 located upstream of the stoppage 20 have remained stopped. The size of the downstream gap 24b, however has grown to four times the fixed pitch because the carriages 14 located downstream of the stoppage 20 continue to advance forward.
As shown in FIG. 2D, when the stopped carriage 14 (carriage No. 0) is restarted, each of the upstream carriages 14 are also immediately restarted so that the fixed operational pitch is maintained. As the carriages 14 forwardly move along the production line 12 after restarting, they continue to move at the fixed operational pitch and the fixed operational speed. The size of the upstream gap 24a has not changed because all of the carriages 14 located upstream of the stoppage 20 restarted together. Therefore, the production loss due to the gap 24a has remained. Accordingly, there is a need in the art for a method of operating an assembly line which reduces production losses such as those due to stoppages along the assembly line.